Film Imperfections Research Articles

Combination of Solid-State and Electrochemical Impedance Spectroscopy To Explore Effects of Porosity in Sol–Gel-Derived BaTiO3 Thin Films

BaTiO3 thin films were deposited onto polycrystalline Pt using a dip-coating technique, with annealing temperatures of 750–900 °C. To avoid film imperfections such as cracking or pinholes, key conditions, including aging periods, water content, and stirring speeds, were refined to produce a pinhole-free, uniform film with some porosity. Whereas those coated a single time short circuited during electrical characterization, this could be avoided in films produced by multiple coating cycles. The effective permittivity of a 600 nm BaTiO3 film was measured at 290 by fitting solid-state impedance data in the frequency range of 100 Hz to 1 MHz. Electrochemical impedance with an aqueous electrolyte allowed evaluation of the porosity, which remained fairly constant between 1 and 5 coating cycles. Using this method, it was possible to estimate the effective permittivity of the BaTiO3 itself as 374 and hence to evaluate the increase in the effective permittivity that could be achieved by minimizing porosity.

Crystal structure development of vanadium oxide thin films deposited by a magnetron sputtering technique

Crystal structure of vanadium oxide films with different thickness, deposited on glass and F:SnO2 coated (FTO) substrates, was studied. It was found that all the films annealed at 450 °C developed a metastable β-V2O5 monoclinic crystal structure, represented by crystals of up to 100 nm. At higher temperatures, the original crystals of relatively thick films of 400 nm on the glass substrate first increased in size and improved their crystal structure symmetry from the monoclinic to tetragonal, then gradually intergrew into spectacular superstructures of crystal “leaves” (500 °C) and “needles” (550 °C). Thin films of 130 nm exhibited a similar crystallization processes but at a higher rate. It was found that the FTO substrate supported the slowest crystal growth dynamics with annealing temperature. The original crystals did not grow in size and retained their low β monoclinic symmetry in the whole temperature range up to 550 °C. The superstructure appeared around film imperfections in the form of plate macroscopic crystals with improved tetragonal symmetry, and it covered only one third of the film surface. The metastable β phase growth and its development were discussed and explained in terms of the combined effect of high crystal/substrate surface tension and crystal surface energy.

Photo-catalytic activity of BiVO4 thin-film electrodes for solar-driven water splitting

There is an ever-increasing attention directed to the development of solar fuels by photo-electrochemical water splitting, given the inexhaustible availability of solar energy. The water oxidation half-reaction is a critical step for the overall water splitting reaction, and the development of suitable photoanodes is therefore required. The present research work focuses on bismuth vanadate thin films’ deposition on FTO glass electrodes, through the dip coating technique, and discusses the influence of the film preparation technique on the electrode's photo-electrochemical performance.The bismuth vanadate thin films were synthesized with thicknesses ranging from 60 to 210nm, depending on a number of dip coatings from 2 to 15. The structural and optical characterization of the films showed that monoclinic scheelite-type phase was obtained in all samples, with crystal sizes ranging from 24 to 65nm, at increasing film thicknesses, and corresponding band gaps between 2.55 and 2.35eV. A maximum photo-current density of about 0.57mAcm−2 at 1.23V vs. RHE under sunlight illumination was obtained for an electrode thickness of 160nm. The electrochemical impedance spectroscopy elucidated the transport mechanisms occurring at the electrolyte–electrode interface, as well as inside the film. The estimation of the equivalent circuit parameters showed that an increasing film thickness decreased the resistance associated to the charge transfer between the electrolyte and the electrode (from 1100 to 450Ω, from 60 to 160nm layer thickness, respectively), given the higher number of active sites involved in the reaction. However, excessive film thicknesses increase the probability of charge recombination within the film and, in the specific case here investigated, can also be associated to film imperfections arising from several deposition-calcination cycles, which further act as traps. These concurring phenomena are of high relevance to isolate the rate-determining step of the water oxidation half-reaction, in the perspective of an optimization of bismuth vanadate film coating on FTO to obtain photo anodes.

Chronoamperometric In Situ Measurement of the Specific Conductivity of a Thin Film on the Disc Electrode Surface

A new method to measure both the total resistance and the specific conductivity of a thin film (for the current flow in the normal direction) is proposed [1].It is assumed that the film has been deposited on the surface of a disc electrode (in static or rotating conditions). A relatively small diameter of the disc is preferable to ensure the maximal uniformity of the film along the surface. The thickness of the film, Lf, should be sufficiently small compared to the disc radius, ro, but it should cover the surface in a uniform manner. The conductivity properties of the deposited material may be isotropic or anisotropic (different in the normal and tangential directions).The primary and secondary distributions of the electric potential and the current after a small-amplitude potential step have been analyzed for such a film-coated electrode in contact with an electrolyte solution, CE and RE being located sufficiently far from the disc electrode (WE). The three-dimensional (3D) character of these distributions inside the solution has been taken into account.Analytical expressions have been derived for the overall short-time resistance of the system, Rtot, with taking into account the contributions of both the solution and the film. Due to the 3D effect this quantity for the primary distribution is not equal to the sum of the short-time resistances of these components of the system, Rs for solution and Rf for the film. Analytical formulas have been deduced, allowing one to calculate immediately the film resistance, Rf, on the basis of measured values of Rtot and Rs. Then, the specific conductivity of the film, kf, is found from relation: kf = Lf / Rf pro 2.Both parameters of the film may be determined by this method for a set of electrode potential values, see Fig.1 for a poly(titanocene-propyl-pyrrole) film.The merits of this procedure to determine the high-frequency transport parameters are: 1) film deposition at a small surface area (but still macroscopic) substrate; 2) no lower restriction for the film thickness (except for their uniformity); 3) in situ measurements inside the conventional cell; 4) non-destructive procedure (film may be used for further texts); 5) simple data treatment; 6) weak effects of local film's imperfections, etc.Support of RFFI (projects 09-03-01172 and 10-08-00847) is acknowledged.Fig. 1 Dependence of the specific conductivity of the poly(titanocene-propyl-pyrrole) film on the potential.[1]. M.A. Vorotyntsev, D.V. Konev / Electrochim. Acta 56 (2011) 9105– 9112

Film properties of low temperature HfO2 grown with H2O, O3, or remote O2-plasma

A reduction of the deposition temperature is necessary for atomic layer deposition (ALD) on organic devices. HfO2 films were deposited by ALD on silicon substrates in a wide temperature range from 80 to 300 °C with tetrakis[ethylmethylamino]hafnium as metal precursor and H2O, O3, or an remote O2-plasma as oxygen source. Growth rate and density were correlated to electrical properties like dielectric constant and leakage current of simple capacitor structures to evaluate the impact of different process conditions. Process optimizations were performed to reduce film imperfections visible at lower deposition temperatures. Additionally, the influence of postdeposition annealing on the structural and electrical properties was studied.

Fundamentals of Selective Ion Transport through Multilayer Polyelectrolyte Membranes

Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K(+)/Mg(2+) selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections. Under MgCl(2) concentration gradients across either (PSS/PAH)(4)- or (PSS/PAH)(4)PSS-coated alumina, transmembrane potentials indicate Mg(2+) transference numbers approaching 0. The low Mg(2+) transference numbers with both polycation- and polyanion-terminated films likely stem from exclusion of Mg(2+) due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nanofiltration, the high asymmetry of membrane permeabilities to Mg(2+) and Cl(-) creates transmembrane diffusion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as -200% for trace monovalent cations such as K(+) and Cs(+). Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of mixed salts.

Stresses from Oxide Film Imperfections During Metal Dusting

When metals or alloys protected by oxide films are exposed to a carburizing atmosphere, carbon may diffuse into the metal substrate because of imperfections in the film. Stresses from the change in carbon concentration in the metal substrate may arise and can result in spallation of the oxide film, crack initiation on the exposed surface and acceleration of metal dusting. In this paper, the stresses at a corner point, where the channel or flaw intersects the interface between the oxide film and metal substrate, are studied by a coupling model between carbon diffusion and elastic deformation. The results show that the stresses of the corner point depend on the oxide film thickness and channel width and are more sensitive to the former. The dependency varies with the time for which the metal substrate is exposed to the carburizing atmosphere. It is expected that the present study may lead to the comprehension of the metal dusting corrosion with regard to the effect of diffusion induced stress.

Reflectance and transmittance of a slightly inhomogeneous thin film bounded by rough, unparallel interfaces

Inhomogeneity of a thin film, rough interfaces of the film with air and substrate and non-parallelism of these interfaces severely affect reflectance and transmittance. Neglecting these film imperfections could give rise to erroneous optical characterisation of the film from measured spectra. Each one of these imperfections, considered alone, has its own characteristic influence on the interference fringes of the reflectance and transmittance spectra. In this work, a model of an inhomogeneous thin film bounded by rough, unparallel interfaces is introduced for calculating reflectance and transmittance at normal incidence. The effects on the spectra due to roughness as predicted by the proposed model are compared with those evaluated by an approach involving the effective-medium approximation (EMA). In this latter approach, a thin transition layer replaces the rough interface, but it proves inadequate for reproducing the transmittance spectrum. Finally, the model is successfully applied to the characterisation of a LiF film thermally evaporated on a glass substrate.

The anodic dissolution of magnesium in chloride and sulphate solutions

The electrochemical behaviour of magnesium was studied in representative chloride and sulphate solutions including NaCl, Na 2SO 4, NaOH and their mixed solutions, HCl, and H 2SO 4: (1) by measuring electrochemical polarisation curves, (2) by using electrochemical impedance spectroscopy (EIS), and (3) by simultaneous measurement of hydrogen gas evolution and measurement of magnesium dissolution rates using inductively coupled plasma atomic emission spectrophotometry (ICPEAS). These experiments showed that a partially protective surface film played an important role in the dissolution of magnesium in chloride and sulphate solutions. Furthermore, the experimental data were consistent with the involvement of the intermediate species Mg + in magnesium dissolution at film imperfections or on a film-free surface. At such sites, magnesium first oxidised electrochemically to the intermediate species Mg +, and then the intermediate species chemically reacted with water to produce hydrogen and Mg 2+. The presence of Cl − ions increased the film free area, and accelerated the electrochemical reaction rate from magnesium metal to Mg +.

Highly asymmetric differential X-ray reflectometry of multilayer films

Bragg scattering of X-ray radiation in a highly asymmetric geometry from Langmuir-Blodgett films was investigated. A complex multi-peak structure was observed in the angular distribution of the scattered X-rays. The proposed simple explanation and the corresponding expressions for the peak positions are in good agreement with the experimental results. The same interpretation is successfully applied to experimental results for multilayer films of a quite different origin, the crystalline W/Si superstructures. Based on this agreement and the generalization of the results obtained we propose a new method of monitoring of angular distribution of X-rays propagating in the film at very small angles. Combined with the expressions obtained, it can be applied, in particular, to the determination of the critical angle of total external reflection from multilayer films which provides additional information compared with a conventional technique. Possibilities to study film imperfections are also outlined.

Bismuth-substituted iron garnet films prepared by RF diode sputtering

Single-phase, magnetically uniaxial Gd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</inf> Bi <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5-y</inf> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> films have been prepared by RF diode sputtering with RF substrate bias on single-crystal (Gd,Ca,Mg) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (Ga,Zr) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> and amorphous substrates. Epitaxial growth is observed by annealing of amorphous films for lattice misfits <0.03Å (tensile stress). Intrinsic properties, such as lattice constant, saturation magnetization, and polar magnetooptic effects, compare well with liquid-phase epitaxial films of equal composition. Film imperfections originating from particles expelled from the ceramic targets cause some optical loss, increased coercivity and increased linewidth in X-ray diffraction.

A Comparison of Thin Film Measurement by Guided Waves, Ellipsometry and Reflectometry

The optical properties of thin dielectric films prepared by vacuum deposition and r.f. sputtering have been measured using guided waves, ellipsometry and reflectometry to assess the relative merits and disadvantages of the three techniques. Particular attention has been given to the effect on these measurements of typical film imperfections, for example refractive index inhomogeneity and anisotropy. The guided wave technique is shown to possess the highest precision of measurement, to be relatively insensitive to film inhomogeneity and to provide the most suitable method for studying film birefringence. Ellipsometry has a high sensitivity of measurement that can be usefully applied to thinner films than those required by the other techniques. The method is well suited for studying inhomogeneity, but because of its sensitivity to this film defect, ellipsometric measurements must always be interpreted with care. Reflectometry, although only of limited accuracy in measuring film properties, is widely applicable and provides values that, in general, are not seriously affected by inhomogeneity.

Propagation distances of surface electromagnetic waves on Ni, Pd, Pt, and W metal surfaces

The propagation distances, L x, of surface electromagnetic waves (SEW) on Ni, Pd, Pt, and W were measured using the two-prism coupling technique at wavelengths from 9.3 to 10.6 microm. Metal samples consisted of films, deposited on glass substrates by electron beam deposition, and metal foils. The measured L x values for the Pd and Pt firms and Pt foil agree to within 20% of the calculated values from the two-media dispersion relation. The measured values of the propagation distance for the Ni film and foil and the W foil agree to within 50% of the calculated L x. Scanning electron microscope (SEM) photographs and Auger analysis indicate that film imperfections and foil surface roughness contributed to the lower propagation distances observed for Pt and Pd while imperfections, roughness, and oxidation contributed to the disagreement between measured and calculated propagation distance for Ni and W.

Investigation of the optical properties of Ag by means of thin semi-transparent films

The optical constants of thin Ag films are determined from measurements of both their transmittance and reflectance. The complex dielectric constant is discussed in terms of intra and interband transitions, in relation with the film crystallographic structure. The optical mass of the conduction electrons is found to be very low, 0·87; their optical relaxation time to depend strongly on the film structure and to be smaller than their electrical relaxation time. The absorption due to interband transitions is considered. A supplementary absorption band related to film imperfections is observed in some cases. Plasma oscillations are also studied in relation with the film quality.